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She said: "The sovereign has legal custody of the minor grandchildren."

"Legislation passed during the reign of George I. It was known as The Grand Opinion for the Prerogative Concerning the Royal Family and it was about the King's control over the education, the raising and the marriage of his grandchildren."

"He did it because he had a very poor relationship with his son, the future King George II, so they had his law passed that meant the King was the guardian of his grandchildren."

The law was passed over 300 years ago in 1717, and during this time the monarch's "right of supervision extended to his grandchildren and this right of right belongs to His Majesty, King of the Realm, even during their father's lifetime".

(Image: Getty)

This means that the Queen also had full custody of a young William and Harry.

The archaic law came to light when Prince Charles and Princess Diana were granted their divorce in 1996.

There was no mention of custody arrangements for their children in the divorce agreement, despite arrangements being planned prior.

This was because neither Charles nor Diana were entitled to full custody as this was granted to the Queen.